Lenore, there is no "formula" about what percentage of a listing price to offer.
Knowing what to offer comes from many places.
* Understanding the true-value of the home based on comps and CMAs
* Knowledge of what your desired target price is
* Understanding the climate of the current market
* An awareness of the list/sale ratio of recent comparable sales
* Negotiating skill (not to put too fine a point on it, but it's much easier to know what to offer, when you'd negotiated for homes before... the more successful times, the better, and you can get a "feel" for the seller, and figure out how they might respond by "profiling" and getting into their mind)
* ... and other intangibles.
As you can see, coming up with that starting point, is a compilation of lots of different things, all coming together to build that price.